I have been working on a project that is only tangentially related to model engineering ... but it did present me with an interesting machining challenge. I got a chance to buy a Powermatic 66 table saw for $200. Those of you who are into woodworking will recognize this as a now discontinued high-end cabinet saw in the same price and quality range as a Delta Unisaw - in other words, $200 was a a killer deal. Of course, the reason it was $200 was because it had seen heavy use in a school setting, but the bearings and trunions are smooth and tight, the motor looks brand new, and the dings and scratches on the table were nothing that concerns me for woodworking.
There were / are three issues that I knew had to be addressed. The first was that motor, a 5hp 3-phase Baldor. The seller told me he knew how to test a 3-phase motor, but he did not think the motor worked. As mentioned, the motor looks brand new; the magnetic switch, on the other hand, looks well used. I figured there might be a problem in the switch, or maybe the motor was wired up for 440 instead of 220, or for a wye instead of a delta, or so on, but in any case I was willing to take a chance on it. When I got the saw home, I checked the wiring - no obvious problems, and it was wired for 220 delta. So I wired a plug on it and plugged it into my 3hp rotary phase converter, hoping it might be enough to start the motor, if the motor actually worked. Wow - the motor starts right up and runs smooth as silk; the magnetic switch functions perfectly. Issue #1 resolved!
The second issue is where the machining challenge came in. The miter gauge slots, especially the left hand slot that gets the most use, was badly worn, unevenly, up to .020" oversize in places. After much pondering, I decided to mill the slots to clean up the wear, leaving each slot parallel, straight, and identical in width - oversize compared to a standard miter slot, but I have a solution for that (see below). The challenge here is that, even though my Bridgeport has a 42" table, it only (only!) has a maximum X-travel of 24" (limited to 22" using the powerfeed), but the slots are 28" long. Meanwhile, the slots are around 16" apart, but the BP has a maximum Y-travel of 12". So ... how to mill both slots, perfectly straight, parallel, and to size across their whole length?
The solution, of course, lies in the flexibility of the Bridgeport design. While the BP design sacrifices rigidity, it allows one to tackle projects that otherwise would be outside the scope of a less flexible machine. I first secured the saw table centered on the mill table and indicated it using the least worn slot; I got it within a half-a-thou or so, not counting the dings and dips. I extended and rotated the ram so that I could reach all the way to one end of the outer slot, which was also the most worn slot. I machined until I cleaned up one side, zeroed the DRO, then machined until I cleaned up the other side, carefully noting the final reading on the DRO. Then I rotated the ram so that I could reach the as-yet unmachined end of the slot. Now came the part that caused a good bit of pre-machining anxiety - cutting this section so as to match exactly with the already-machined section. I marked the already machined part with marker; then I used a paper shim to set the cutter to within a couple of thou to this already machined part. I then milled the un-machined section a half-thou at a time, extending into the previously machined part until I just scraped the marker off. At that point, I zeroed the DRO and moved to a couple of thou short of the recorded DRO reading on the other side of the slot, then again worked my way in half-a-thou at a time, using both the DRO and the marker to hit the size. I also used an adjustable parallel set to the width established in the previously machined end of the slot as a gauge; this was a very helpful sanity check. Once I had both ends of the slot machined straight and parallel and to size, with the adjustable parallel sliding smoothly but snugly throughout with the same "feel," I wound the ram back in and repeated the whole thing with the inner slot, opening it to the same size, again creeping up on the size and using the adjustable parallel to check.
I have to admit that I was both relieved and more than a little pleased with myself when I got done. I had feared making a mess of it, but as best I can tell, I succeeded to within the limits of my ability to measure. Whew!
Of course, this now means that a standard miter gauge will no longer fit snugly in the slots. But that brings up the third issue, which is that the saw lacked a miter gauge. The solution to both problems, of course, is to make my own, or at least make my own bar, custom sized to my now over-sized miter gauge slots. That is the next project awaiting my attention.
And that is "what I have been doing today" (or rather, over a good many days)!
